1. Field of the Invention
The present invention relates to an inking roller in the form of a screen roller for an offset inking mechanism or similar ink applying means.
2. Prior Art
An inking mechanism of the present type can only fulfill the high demands with regards to the reproducibility of the printing form, and the imaging accuracy on the print carrier, by maintaining constant and correct tonal quality, if it is possible to ensure over a sufficiently long period equally thick ink coatings for the roller transferring the ink. Inking rollers cooperating with a doctor blade which have been provided by the prior art do not generally simulataneously fulfill all the requirements. It has been known in the prior art to scrape excess ink off of the surface of a roller using a doctor blade, leaving ink in depressions formed in the surface of the roller.
As the edge of the doctor blade sufficiently hardened for this purpose slides over the screen roller surface, it must be ensured that the necessarily occurring abrasion remains negligible, particularly if the surface provided for trapping ink with a matrix of cells only has very narrow webs between the cells and consequently there is a small web-cell ratio.
It is known in this connection to apply a preferably hard ceramic coating to the dynamically balanced core of such a roller or to the spherical surface thereof and to engrave the necessary number and shape of the cells or depressions into such a surface by means of a laser beam. These known rollers known as Laserlox with standard doctor blade use give more than 200 million running metres without any detectable wear and therefore without any significant change to the volume absorption for the cells. However, it is considered inadequate that the hydrophilic ceramic material can lead to problems in transferring the damping agent, so that the actual ink impression has a watered down appearance.
Inking mechanisms operating with highly viscous printing inks, such as is, e.g., the case with offset printing presses, make use of a dip roller dipped into an ink pan, an inking roller inking a printing form applied to a cylinder and a transfer roller transferring the ink from the dip roller to the inking roller, the transfer roller with the ink trapping depressions and the interposed webs bearing an image in a halftone or screen arrangement. In order to be able to guarantee a uniformly thin ink film, so-called ink storage or conditioning rollers are associated with the transfer roller. For bringing about a dosed ink transfer, it is conventional practice to provide control means, which can be manually adjusted for each individual case, which not only require increased construction costs, but also complicated operation.
Screen rollers for an offset inking mechanism with cells defined by webs which can be scraped are known, and have an outer surface made from a ceramic material, in which the cells have a hydrophobic lining (DE-OS 37 13 027). The prior art screen roller, which in the known arrangement is made from steel, cooperates with an inking roller to which, besides the ink, is supplied a damping agent. This again creates a risk that the damping agent required for wetting the printing plate passes via the inking roller to the screen roller and has a negative influence on the adhesion of the ink in the cells, and therefore on the filling of the latter with ink, so that there can be fluctuations in the ink quantity and density. For this reason, particular attention is paid to the hydrophobic lining of the cells. In connection with a multilayer roller construction, attention is paid to an optimum material selection for each individual layer and aluminum oxide or chromium oxide has been found to be a particularly advantageous wear-resistant material for the hard ceramic coating, which can be engraved by means of a laser beam. Following engraving, the hydrophobic material layer is applied in the form of a uniformly thin copper layer with a thickness of 20 to 50 .mu. in such a way that, on the one hand, high wear resistance through the webs made from hard ceramic material is ensured, and, on the other, the high affinity of this ceramic material for water, which is disadvantageous for printing, can be avoided in the vicinity of the cells by a thin evaporation coated copper layer. An initially very thinly evaporation coated copper film 4 to 5 .mu. thick can be subsequently brought to the aforementioned thickness galvanically, the copper film then passing over the complete screen surface of the inking roller. The initially uniform copper film can, if desired, subsequently be ground from the vicinity of the hard ceramic web surface prior to the use of the roller.
In this known inking roller, the shape and size of the halftone screening of the ceramic layer necessarily determines the volume of the ink metering cells, the web width of the engraving in the ceramic surface serving as a further ink volume control.
It is worth mentioning in this connection that such multilayer rollers are also known in such a form that the radially external oxide layer to the roller axis has the same thickness approximately everywhere in the vicinity of the cells. Preferably aluminum is used as the carrier material and alumina as the hydrophobic coating. The depressions in the screen roller surface along the base face and in the vicinity of the webs between the cells have higher oxide coatings than along the sloping cell walls, here again the lining layer being produced in the galvanic oxidizing bath (DE-OS 36 15 141).